Chapter 4: Wave equations

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# Chapter 4: Wave equations - PowerPoint PPT Presentation

Chapter 4: Wave equations. What is a wave?. what is a wave?. anything that moves. This idea has guided my research: for matter, just as much as for radiation, in particular light, we must introduce at one and the same time the corpuscle concept and wave concept. Louis de Broglie.

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what is a wave?

anything that moves

This idea has guided my research:

for matter, just as much as for radiation, in particular light, we must introduce at one and the same time the corpuscle concept and wave concept. Louis de Broglie

Waves move

f(x)

t1 > t0

t2 > t1

t3 > t2

t0

v

x

0 1 2 3

• A wave is…
• a disturbance in a medium
• -propagating in space with velocity v
• -transporting energy
• -leaving the medium undisturbed

1D traveling wave

f(x)

t1 > t0

t2 > t1

t3 > t2

t0

v

x

0 1 2 3

To displace f(x) to the right: xx-a, where a >0.

Leta = vt, where v is velocityand tis time

-displacement increases with time, and

-pulse maintains its shape.

So f(x -vt)represents a rightward, or forward, propagating wave.

• f(x+vt)represents a leftward, or backward, propagating wave.

The wave equation

• Let y =f(x'), where x' = x ± vt. So and

Now, use the chain rule:

So Þ and Þ

Combine to get the 1D differential wave equation:

works for anything that moves!

Harmonic waves

• periodic (smooth patterns that repeat endlessly)
• generated by undamped oscillators undergoing harmonic motion
• characterized by sine or cosine functions
• -for example,
• -complete set (linear combination of sine or cosine functions
• can represent any periodic waveform)

Snapshots of harmonic waves

at a fixed time:

at a fixed point:

T

A

A

propagation constant: k = 2p/l

wave number:k = 1/l

frequency: n = 1/T

angular frequency:w = 2pn

n≠ v

v = velocity (m/s)

n = frequency (1/s)

v = nl

Note:

The phase of a harmonic wave

Thephase, j, is everything inside the sine or cosine (the argument).

y = A sin[k(x ± vt)]

j= k(x ± vt)

For constant phase, dj = 0 = k(dx ± vdt)

which confirms that v is the wave velocity.

p

Absolute phase

• y = A sin[k(x ± vt) + j0]

A = amplitude

j0 = initial phase angle (or absolute phase) at x = 0 and t =0

How fast is the wave traveling?

The phase velocity is the wavelength/period: v = l/T

Since n = 1/T:

In terms of k, k = 2p/l, and

the angular frequency, w = 2p/T, this is:

v = ln

v = w/k

Phase velocity vs. Group velocity

Here, phase velocity = group velocity (the medium is non-dispersive).

In a dispersive medium, the phase velocity ≠ group velocity.

Complex numbers make it less complex!

P = (x,y)

x: real andy: imaginary

P = x + i y

• = A cos(j) + i A sin(j)

where

Euler’s formula

“one of the most remarkable formulas in mathematics”

Links the trigonometric functions and the complex exponential function

eij = cosj + i sinj

so the point,P = Acos(j) + iA sin(j), can also be written:

P = A exp(ij) = Aeiφ

where

A = Amplitude

j = Phase

Harmonic waves as complex functions

Using Euler’s formula, we can describe the wave as

y = Aei(kx-wt)

so that y = Re(y) = A cos(kx – wt)

• y = Im(y) = A sin(kx – wt)

~

~

~

Why?

Math is easier with exponential functions than with trigonometry.

Plane waves

A plane wave’s contours of maximum field, called wavefronts, are planes. They extend over all space.

The wavefronts of a wave sweep along at the speed of light.

• equally spaced
• separated by one wavelength apart
• perpendicular to direction or propagation

Usually we just draw lines; it’s easier.

Wave vector k

represents direction of propagation

• Y = A sin(kx – wt)

Consider a snapshot in time, say t = 0:

• Y = A sin(kx)
• Y = A sin(krcosq)
• If we turn the propagation constant (k = 2p/l) into a vector, krcosq = k · r
• wave disturbance defined by r
• propagation along the x axis
• Y = A sin(k · r – wt)

General case

• k · r = xkx = yky + zkz
• = krcosq
•  ks

arbitrary direction

In complex form:

Y = Aei(k·r - wt)

Substituting the Laplacian operator:

Spherical waves

• harmonic waves emanating from a point source
• wavefronts travel at equal rates in all directions

Derivation of the wave equation from Maxwell’s equations

E and B are perpendicular

Perpendicular to the direction of propagation:

I. Gauss’ law (in vacuum, no charges):

so

II. No monopoles:

Always!

so

Always!

E and B are perpendicular

Perpendicular to each other:

so

Always!

E and B are harmonic

Also, at any specified point in time and space,

where c is the velocity of the propagating wave,

EM wave propagation in homogeneous media

• The speed of an EM wave in free space is given by:
• 0= permittivity of free space, 0= magnetic permeability of free space
• To describe EM wave propagation in other media, two properties of the medium are important, its electric permittivity ε and magnetic permeability μ. These are also complex parameters.
•  = 0(1+ ) + i= complex permittivity
• = electric conductivity
• = electric susceptibility (to polarization under the influence of an external field)
• Note that ε and μ also depend on frequency (ω)

x

z

y

Simple case— planewave with E field along x, moving along z:

General case— along any direction

which has the solution:

where

and

and

= wave vector

Plane of constant , constant E

Arbitrary direction

x

z

y

Electromagnetic waves transmit energy

Energy density (J/m3) in an electrostatic field:

Energy density (J/m3) in an magnetostatic field:

Energy density (J/m3) in an electromagnetic wave is equally divided:

Rate of energy transport

Rate of energy transport: Power (W)

A

cDt

Power per unit area (W/m2):

Poynting

Pointing vector

Poynting vector oscillates rapidly

Take the time average:

Light is a vector field

A 2D vector field assigns a 2D vector (i.e., anarrow of unit length having a unique direction) to each point in the 2D map.

Light is a 3D vector field

A light wave has both electric and magnetic 3D vector fields:

A 3D vector field assigns a 3D vector (i.e., anarrow having both direction and length) to each point in 3D space.

Polarization

• corresponds to direction of the electric field
• determines of force exerted by EM wave on charged particles (Lorentz force)
• linear,circular, eliptical

Evolution of electric field vector

circular polarization

y

x

Exercises

You are encouraged to solve all problems in the textbook (Pedrotti3).

The following may be covered in the werkcollege on 14 September 2011:

Chapter 4:

3, 5, 7, 13, 14,

17, 18, 24